Impact of timeordered measurements of the two states in a niobium superconducting qubit st.pdf

Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure K. Segall, D. Crankshaw, D. Nakada, T.P. Orlando, L.S. Levitov, S. Lloyd Massachusetts Institute of Technology, Cambridge, MA 02139 N. Markovic, S.O. Valenzuela, M. Tinkham Department of Physics, Harvard University, Cambridge, MA 02138 K.K. Berggren Group 86, MIT Lincoln Laboratories, Lexington, MA 02421 Abstract : Measurements of thermal activation are made in a superconducting, niobium Persistent-Current (PC) qubit structure, which has two stable classical states of equal and opposite circulating current. The magnetization signal is read out by ramping the bias current of a DC SQUID. This ramping causes time-ordered measurements of the two states, where measurement of one state occurs before the other. This time-ordering results in an effective measurement time, which can be used to probe the thermal activation rate between the two states. Fitting the magnetization signal as a function of temperature and ramp time allows one to estimate a quality factor of 106 for our devices, a value favorable for the observation of long quantum coherence times at lower temperatures. 1 The concept of thermal activation of a particle over an energy barrier plays a critical role understanding many problems in condensed matter physics. Starting with Kramers,1 expressions for the thermal activation rate have been derived in both the low and high damping regimes.2 These expressions are often applied to analyses of Josephson junction circuits, where the particle coordinate represents the phase difference of the superconducting order